The purpose of this proposal is to define the influence of microRNA expression on specific changes in endothelial cell function that occur in response to shear stress forces. Shear stress forces, generated by blood flow, play an important role in the regulation of vascular tone, vascular remodeling, and the focal development of atherosclerotic lesions. In the arterial tree, endothelial cells are exposed to different shear stress forces that induce distinct effects on gene expression and function. Unidirectional shear stress, which occurs in the straight part of the tree, elicits a change in endothelial gene expression that is generally anti-inflammatory and atheroprotective. In contrast, oscillatory shear stress, which occurs at branch points in the arterial tree, induces an overall pro-inflammatory and proatherosclerotic response. MicroRNAs (miRNAs) are a recently recognized class of short (19-25 nt), single stranded, noncoding RNAs that have become a major focus in molecular biology research because they posttranscriptional regulate the expression of genes involved in an array of cell functions, including differentiation, growth, proliferation, and apoptosis. Although an important role for miRNA expression has been demonstrated for various biological processes, including cardiogenesis and angiogenesis, data on the role of specific miRNAs in endothelial cell biology is currently limited. In preliminary studies of human endothelial cells subjected to prolonged unidirectional shear stress (24 hrs, 15 dynes/cm2), a group of miRNAs was identified whose expression was significantly upregulated in response to this stimulus, suggesting that these miRNAs are important in regulating gene expression and function in endothelial cells. To further define the role of miRNA expression in modulating shear stress-induced changes in endothelial cell biology, the function of one highly shear-responsive miRNA, miR-155, will be studied. Specifically, the proposed research will define the impact of miR-155-target gene interaction on endothelial cell apoptosis, barrier function and migration. To study the mechanism by which miR-155 modulates apoptosis, we will focus on the SHIP1/PI3K/Akt pathway. To study the mechanism by which miR-155 modulates endothelial monolayer permeability and migration, we will focus on the RhoA/Rho kinase pathway. We will test the influence of miR- 155 on these critical pathways by experimentally manipulating expression of miR-155, its target gene, or members of the pathway that are downstream of the miRNA-target gene interaction. Subsequently, the effect of these manipulations on endothelial cell apoptosis, monolayer permeability, and migration will be quantified. Finally, the association between shear-induced miR-155 expression and activity of endothelial cell regulatory pathways will be studied in vivo, in a mouse model of altered aortic flow. We anticipate that these studies will help address a deficit in our knowledge about the function of miRNAs in endothelial cells and will enhance our understanding of the mechanisms by which shear stress forces modulate vascular disease.